Wellbore fluids serve many important functions throughout the process in drilling for oil and gas. One such function is cooling and lubricating the drill bit as it grinds though the earth's crust. As the drill bit descends, it generates “cuttings,” or small bits of stone, clay, shale, or sand. A wellbore fluid serves to transport these cuttings back up to the earth's surface. As drilling progresses, large sections of pipe called “casings” are inserted into the well to line the borehole and provide stability. One of skill in the art should appreciate these uncased sections of the borehole, which are exposed to the high pressures of the reservoir, must be stabilized before casing can be set; otherwise, a reservoir “kick” or, in the extreme case, a “blowout”—a catastrophic, uncontrolled inflow of reservoir fluids into the wellbore—may occur. A wellbore fluid, if monitored properly, can provide sufficient pressure stability to counter this inflow of reservoir fluids.
A critical property differentiating the effectiveness of various wellbore fluids in achieving these functions is density, or mass per unit volume. The wellbore fluid must have sufficient density in order to carry the cuttings to the surface. Density also contributes to the stability of the borehole by increasing the pressure exerted by the wellbore fluid onto the surface of the formation downhole. The column of fluid in the borehole exerts a hydrostatic pressure (also known as a head pressure) proportional to the depth of the hole and the density of the fluid. Therefore, one can stabilize the borehole and prevent the undesirable inflow of reservoir fluids by carefully monitoring the density of the wellbore fluid to ensure that an adequate amount of hydrostatic pressure is maintained.
It has been long desired to increase the density of wellbore fluids, and, not surprisingly, a variety of methods exist. One method is adding dissolved salts such as sodium chloride, calcium chloride, and calcium bromide in the form of an aqueous brine to wellbore fluids. Another method is adding inert, high-density particulates to wellbore fluids to form a suspension of increased density. These inert, high-density particulates often are referred to as “weighting agents” and typically include powdered minerals of barite, calcite, or hematite.
Naturally occurring barite (barium sulfate) has been utilized as a weighting agent in drilling fluids for many years. Drilling grade barite is often produced from barium sulfate containing ores either from a single source or by blending material from several sources. It may contain additional materials other than barium sulfate mineral and thus may vary in color from off-white to grey or red brown. The American Petroleum Institute (API) has issued international standards to which ground barite must comply. These standards can be found in API Specification 13A, Section 2.